1. Field of the Invention
The present invention relates to a connection unit provided between an optical amplifier and optical cable, an optical fiber line unit accommodated in an optical cable, an optical cable, and an optical transmission system including an optical cable and optical amplifier.
2. Related Background Art
In a submarine optical cable or the like, to transmit optical signals over a long distance, repeaters are provided at predetermined distance intervals (about 90 km at maximum; about 50 km in general) to amplify a weakened optical signal.
An optical cable has a plurality of optical fiber lines in an optical cable sheath portion, and the plurality of optical fiber lines are respectively connected to a plurality of optical fibers extending from an optical amplifier (to be also referred to as an optical amp hereinafter) mounted in each repeater. A plurality of optical signals propagating through the plurality of optical fiber lines collectively strike the optical amp via the plurality of optical fibers extending from the optical amp to be collectively amplified.
The present inventors have found the following problems upon examining the above conventional technique. An optical amp has a so-called gain tilt property, i.e., the property of having different gains depending on the wavelengths of incident optical signals. This gain tilt changes depending on the power of an incident optical signal. FIG. 11 shows how the gain tilt changes depending on the incident power. Referring to FIG. 11, reference symbol P1 denotes the direction in which the incident power decreases; and P2, the direction in which the incident power increases. As shown in FIG. 11, when the incident power is low, the gain on the long-wavelength side is higher than that on the short-wavelength side, and vice versa.
In this case, signals having various wavelengths are transmitted through each optical fiber line owing to WDM (Wavelength Division Multiplex) transmission. Consequently, signals vary in power depending on the wavelengths due to the gain tilt, and the transmission characteristics deteriorate.
In this case, it is relatively easy to correct a gain tilt with respect to one incident light beam. However, optical signals propagating through the respective optical fiber lines are collectively incident on the optical amp. Since the respective incident optical signals have propagated through a plurality of optical fiber lines with different transmission losses (e.g., losses in the optical fibers themselves or losses due to side pressures and bending), the powers of the respective optical signals differ from each other. If the deviations among transmission losses in the respective optical fiber lines are large, the deviations among the powers of the incident light beams are large. Therefore, gain tilts with respect to a plurality of optical signals vary, and it is difficult to suppress the gain tilts with respect to all optical signals to a predetermined value or less, resulting in a deterioration in transmission characteristics.
In a submarine optical cable, in particular, since optical amplification is performed by optical amps on the respective optical fiber lines at predetermined distance intervals, if the deviations among transmission losses in the respective optical fiber lines are large, the gain tilt increases every time each optical signal passes through an optical amp. This greatly affects the transmission characteristics.
It is therefore an object of the present invention to provide a connection unit, optical fiber line unit, optical cable, and optical transmission system which can improve transmission characteristics.
A connection unit according to the present invention is provided between a plurality of first optical fibers extending from an optical amplifier and a plurality of second optical fibers in an optical cable. The connection unit comprises at least one optical component to connect one first optical fiber selected from the plurality of first optical fibers to one second optical fiber selected from the plurality of second optical fibers. Each of the optical components comprises a third optical fiber that can be connected to the selected first optical fiber with a low connection loss, a fourth optical fiber that can be connected to the selected second optical fiber with a low connection loss, and a loss adjusting section for adjusting a transmission loss of an optical transmission line including the selected first and second optical fibers. The connection unit adjusts a difference between a highest transmission loss and a lowest transmission loss among transmission losses in a plurality of optical transmission lines each including one of the plurality of first optical fibers and one of the plurality of second optical fibers to not more than 0.005 dB/km.
Since the connection unit can adjust the deviation among the transmission losses in a plurality of optical transmission lines to 0.005 dB/km or less, the powers of a plurality of optical signals incident on the optical amplifier can be almost equalized. This makes it possible to suppress the influences of the gain tilts and improve the transmission characteristics.
The connection unit according to the present invention may be characterized in that the loss adjusting section of the optical component includes a connection portion between the third and fourth optical fibers, and the transmission loss is adjusted by a connection loss at the connection portion.
The connection unit according to the present invention may be characterized in that the third and fourth optical fibers of the optical component are fusion-spliced to each other at the connection portion, and a central axis of a core portion of the third optical fiber is offset from a central axis of a core portion of the fourth optical fiber at the connection portion.
The connection unit according to the present invention may be characterized in that the third and fourth optical fibers of the optical component are fusion-spliced to each other, and a core portion of at least one of the third and fourth optical fibers is enlarged at the connection portion.
The connection unit according to the present invention may be characterized in that the loss adjusting section of the optical component includes a bent portion formed on at least one of the third and fourth optical fibers, and the transmission loss is adjusted by a bending loss at the bent portion.
The connection unit according to the present invention may be characterized in that the loss adjusting section of the optical component includes an optical attenuator, and the transmission loss is adjusted by the optical attenuator.
An optical fiber line unit according to the present invention comprises a plurality of optical fibers accommodated in an optical cable sheath portion and constituting an optical cable together with the optical cable sheath portion, each of the optical fiber lines having a length of not less than 30 km. Each of the optical fiber lines is formed by connecting a plurality of optical fibers. A difference between a highest transmission loss and a lowest transmission loss among transmission losses in the optical fiber lines is not more than 0.005 dB/km.
According to this optical fiber line unit, since the difference between transmission losses in a plurality of optical fiber lines is 0.005 dB/km or less, the powers of a plurality of optical signals incident on the optical amplifier are almost equalized. This makes it possible to suppress the influences of gain tilts and improve the transmission characteristics.
An optical cable according to the present invention is constituted by a plurality of optical transmission lines each including an optical fiber line having a first mode field diameter. At least one of the plurality of optical transmission lines includes an optical fiber connected to one end of the optical fiber line and having a second mode field diameter smaller than the first mode field, and a loss adjusting section for adjusting an overall transmission loss in the selected optical transmission line including the optical fiber line and the optical fiber. A difference between a highest transmission loss and a lowest transmission loss among transmission losses in the plurality of optical transmission lines is not more than 0.005 dB/km.
An optical cable according to the present invention is constituted by a plurality of optical transmission lines each including an optical fiber line having a portion having a first mode field diameter and a portion having a second mode field diameter larger than the first mode field diameter. At least one optical transmission line selected from the plurality of optical transmission lines includes an optical fiber connected to one end of the optical fiber line and having a third mode field diameter larger than the first mode field diameter and smaller than the second mode field diameter, and a loss adjusting section for adjusting an overall transmission loss in the selected optical transmission line including the optical fiber line and the optical fiber. A difference between a highest transmission loss and a lowest transmission loss among transmission losses in the plurality of optical transmission lines is not more than 0.005 dB/km.
In this optical cable, since the deviation between the transmission losses in a plurality of optical transmission lines is 0.005 dB/km or less, the powers of a plurality of optical signals incident on the optical amplifier are almost equalized to suppress the influences of gain tilts, and the transmission characteristics can be improved.
An optical transmission system according to the present invention comprises an optical cable having a plurality of optical fiber lines, an optical amplifier, and a plurality of optical fibers extending from the optical amplifier. At least one of optical transmission lines including one optical fiber line selected from the plurality of optical fiber lines and one optical fiber selected from the plurality of optical fibers includes a loss adjusting section for adjusting a difference between a highest transmission loss and a lowest transmission loss among transmission losses in the plurality of optical transmission lines to not more than 0.005 dB/km.
In this optical transmission system, since the deviation between the transmission losses in a plurality of optical transmission lines can be adjusted to 0.005 dB/km or less, the powers of a plurality of optical signals incident on the optical amplifier are almost equalized to suppress the influences of gain tilts, and the transmission characteristics can be improved.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.